Tat is a small HIV regulatory protein essential for viral replication whose well-known function is to enhance transcription elongation from the viral promoter. Tat recruits the host elongation factor P-TEFb to TAR RNA to phosphorylate the CTD of RNAP II. The activity of the Cdk9 kinase of P-TEFb is inhibited by the 7SK snRNP, and we previously found that the inhibited complex is recruited to the HIV promoter during transcription initiation and is ejected when TAR is synthesized thereby activating the kinase. Another set of host factors, the super-elongation complex (SEC), operates later to further enhance Tat-mediated elongation. There has been substantial recent progress on structural studies of Tat and its complexes, most notably Tat bound to P-TEFb and components of the SEC. It is clear that Tat is a largely disordered protein that requires the P-TEFb template for its folding, primarily through interactions with the cyclin T1 subunit. Despite this progress, the transcription process is quite complex and dynamic and many other host factors come into play. Based on our previous HARC proteomic studies, we have recently identified several new Tat cofactors important for its function. One factor, PJA2, directly ubiqutinates lysines on Tat in a non-degradative manner to stimulate its activity. Another set of three factors ? ZFP91, UBE2O, and NAP1L4 ? forms a cytoplasmic complex (ZUN) that enhances Tat activity via ubiquitination of HEXIM1, a subunit of the 7SK snRNP. This non-degradative modification, carried out by the UBE2O ligase, helps disassemble the inhibitory 7SK complex and releases PTEFb to relocalize from the cytoplasm to the nucleus. Based on these novel findings, we will further our structural studies of Tat and host complexes by: 1) examining ubiquitination of Tat by PJA2 and determining structures of relevant complexes; 2) determining structures of Tat with novel 7SK snRNP complexes; and 3) examining connections between 7SK snRNP complexes and the SEC. The discovery of the PJA2 and ZUN complexes illustrates another way in which HIV has hijacked the host ubiquitination machinery to enhance its replication. Advancing structural knowledge of these new Tat transcription complexes will illuminate their mechanisms of action not only for the virus but also for host cell transcription.